Kinetic Friction | Dynamic Friction | Sliding Friction Kinetic friction, also known as dynamic or sliding friction occurs when a body is in relative motion with another, with their surfaces in contact. It is the frictional force that slows down, and eventually stops a moving object. For example, a soccer ball given a slight kick will begin to roll on the pitch, then slows down, and then come to a stop. The ball is slowed down, and then stopped after it has started moving with a velocity, as a result of the kinetic friction between its surface and the pitch. Without confusing the two, the difference between static friction and kinetic friction is that static friction occurs when the object is stationary, that is, not moving, while kinetic friction takes place when the object has started moving. It is kinetic friction that makes sure an object does not continue to move forever when a force has acted on it as stated in Newton’s first law of motion . As is the case with static friction, kinetic friction depends on the nature of the materials or surfaces in contact, and the magnitude of the force that set the body in motion. However, kinetic friction is less than static friction for the same material surfaces in contact.

Kinetic friction can be demonstrated in the laboratory by the following experiment: A wooden block placed on a wooden table is given a push to move it. It moves forward for a while, slows down and come to a halt. It slows down because of the kinetic friction that acted on its velocity, in the opposite direction to its motion, which eventually brings it to a stop. To offset the existing kinetic friction between the wooden surfaces of both the block and the table, additional forward force is required. This is made possible by attaching a string to the block, and weights attached to the other end of the string. With the aid of a pulley system as shown below, the attached weights generate a forward force on the block making it to continue moving. By attaching the right weights on the string, the block will continue moving at constant velocity. At this point the net force acting on the block is zero. The kinetic friction acting between the surfaces of two bodies in relative motion to each other is represented by the formula: Physics 101 Example Exam IV (d) a change in momentum 3. Which of the following is true about momentum?

(a) it is a vector (b) it is a product of mass times velocity (c) impulses are required to change it (d) all of the above (e) none of the above 4. A car with a mass of 2000 kg moves at 10 m / s. Calculate the braking force needed to bring the car to a halt in 5 seconds. (a) 100000N (c) 20000N (d) 40 N 5. The car moving at 50 kilometers/hour skids 10 meters with locked brakes. How far will the car skid with locked brakes if it is traveling at 150 kilometers/hour? (a) 20 meters (d) 120 meters (e) 180 meters 6. At what point in its motion is the kinetic energy of a pendulum bob a minimum? (a) at the highest points in its motion (b) at the lowest point in its motion (c) half way between the highest and lowest points in its motion 7. A 1-kg object rests on a table 1 m above the floor.

What is the gravitational potential energy in Joules of the object with respect to the floor? (a) 1 (b) 9.8 (c) 32 (d) 11.8 (e) 0 8. When a car is braked to a stop, its kinetic energy is transformed to (a) stopping energy. (c) heat energy. (d) energy of rest. 9. If you push on an immovable wall for an hour, is there any work done on the wall? (a) Yes (b) No 10. Can a body have kinetic energy without having momentum? (a) Yes (b) No 11. A torque acting on an object tends to produce (a) equilibrium. (c) linear motion. (d) stability. 12. A coin and a ring roll down an incline starting at the same time from rest. The one to reach the bottom first will be the (a) ring (c) both reach the bottom at the same time 13. The center of mass of a body can be outside of the body (a) True (b) False 14.

When a spinning ice skater draws her arms in close to her body, she starts to spin faster. This is an example of (a) external torque friction (c) conservation of angular momentum (d) spin impulse 15. Which is the larger force? (a) the force that the earth exerts on the moon (b) the force that the moon exerts on the earth (c) neither 16. A merchant of gold should_________ at a high altitude and ___________ at a low altitude. (a) buy, sell (c) buy or sell, buy or sell 17. When the distance between two stars decreases by half, the force between them (a) decreases by one-quarter (c) increases to twice as much (d) increases to four times as much (e) stays the same 18. The planet Jupiter is about 300 times as massive as earth, yet on its surface you would weight only about 3 times as much. This is because (a) your mass is 100 times less on Jupiter. (b) Jupiter is significantly farther from the sun. (c) Jupiter's radius is 10 times the earth's radius.

(d) you are 100 times more weightless there. (e) none of these. 19. Inside a freely-falling elevator, there would be no (a) gravitational force on you. (b) apparent weight for you. (c) both of these 20. Which is most responsible for the ocean tides? (a) ships (c) the moon (d) the sun 21. Padded dashboards in cars are safer in an accident than nonpadded ones because an occupant hitting the dash has (a) increased time of impact. (b) decreased impulse. (d) a and b above. (e) a and c above. 22. Two identical gliders slide toward each other on an air track. One moves at 1 m/s and the other at 2 m/s. They collide and stick. The combined mass moves at (a) 3/4 m/s. (d) all of these (e) none of these. 25. When you are in the way of a fast-moving object and can't get out of its way, you will suffer a smaller force of impact if you decrease its momentum over a (a) short time. (b) long time. (c) same way either way.

26. When you jump from an elevated position you usually bend your knees upon reaching the ground. By doing this, you make the time of the impact about 10 times as great as for a stiff-legged landing. In this way the average force your body experiences is (a) less than 1/10 as great. (b) about 1/10 as great. (c) about 10 times as great. (d) more than 1/10 as great. 27. If you push an object with twice the work input for twice the time, your power input is (a) four times as much. (b) twice. (c) the same amount as for half the work in half the time. 28. Whereas impulse involves the time that a force acts, work involves the (a) time and distance that a force acts. (b) distance that a force acts. (c) acceleration that a force produces. 29. An object that has kinetic energy must be (a) falling. (c) at an elevated position. (d) at rest. 30. Do 100 J of work in 50 s and your power output is (a) 1/4 W.

(d) 4 W. (e) more than 4 W. 31. Put a pipe over the end of a wrench when trying to turn a stubborn nut on a bolt, to effectively make the wrench handle twice as long, you'll multiply the torque by (a) two. (b) four. (c) eight. 32. A car travels in a circle with constant speed. The net force on the car is (a) zero because the car is not accelerating. (b) directed towards the center of the curve. (c) directed forward, in the direction of travel. (d) none of these. 33. The reason the moon does not crash into the earth is that the (a) moon has less mass than the earth. (b) earth's gravitational field is weak at the moon. (c) gravitational pull of other planets keeps the moon up. (d) moon has a sufficient tangential speed. (e) none of these. The science of swimming - Explain that Stuff The science of swimming Tweet by Chris Woodford . Last updated: March 16, 2016. Humans evolved from sea creatures but—looking at our bodies—you'd never know it. We couldn't be less well suited to moving through water if we tried.

We don't float too well, can't breathe for long beneath the surface, and rapidly tire as we thrash through the waves trying to move ourselves along; in a straight race with a dolphin or a shark, you'll always come last! But there's one big advantage we humans do have: we know about science. We understand how forces work and how to use them to our advantage. If you've never thought about swimming as a science, now's the time to start. Apply some scientific thinking and you'll find you can swim much more effectively. If you're a nervous nonswimmer, thinking about the solid science that keeps people afloat can give you enough confidence to break through your fear. So what are we waiting for? Let's take the plunge—with a closer look at the science of swimming! Photo: Swimming takes humans back from the land to the ocean—or the pool! You have to apply forces to move yourself through the water and other forces slow you down. Understand those forces and you can swim much more effectively. Photo by R. Jason Brunson courtesy of US Navy . What is swimming? That sounds like a trivial question, but it helps to be clear.

Swimming is moving your body through water (a moderately viscous fluid) that's either still (as in a swimming pool), turbulent (as in the ocean), or somewhere in between. If you're swimming completely under the surface (for example, scuba diving), you're moving through relatively still water; other times, you're going to be moving along at the more turbulent interface between air and water, with your legs, arms, head, and body moving from one element to the other and back again, speeding up or slowing down as they cross the border. Photo: Even the best swimmers have to move along the choppy interface between air and water. It's the most inefficient place to swim, but the only place you can do it if you need to breathe air! Photo by Jennifer A. Villalovos courtesy of US Navy . Water versus air Before we can understand the science of swimming, it helps to remember that air (a gas) is very different from water (a liquid). The biggest difference is that water is much more dense (the same volume of it weighs much more) and viscous (in other words, thicker—in the same way that treacle is more viscous than water).

The difference between air and water makes a huge difference to how we can move on air and land. When you walk on land, the main thing your body has to do is work against gravity (lifting your legs, swinging your arms, and keeping you from toppling over through constant adjustments of your balance) and a little bit of friction where your shoes meet the ground. If you move more quickly (say, on a bicycle ), air resistance becomes a more important force than gravity; unless you're walking into a really strong wind, you barely notice the air while you're walking. When you're in the water, gravity is much less important because your buoyancy (tendency to float) largely cancels it out. The main force you have to think about as a swimmer is drag—water resistance. We'll come to that in a moment. Other differences between water and air are important if you swim outdoors, particularly in the winter months: because water is much more dense than air, it removes heat from your body about 25–40 times faster than air at the same temperature.

(That's why surfers and "wild" outdoor swimmers tend to wear wetsuits to avoid hypothermia, the very dangerous cooling of the body's core that can kill you.) Because water is so dense, it takes a long time to warm up. That's why the ocean temperature typically lags behind the land temperature by 2–3 months in countries such as the East Coast of the United States and the UK (where the ocean is often warmest in September). Newton's laws of swimming If you love science but swimming scares you, you'll find it very helpful—as I did when I was learning to swim—to think about Newton's three laws of motion . Among the most fundamental rules of physics, these three basic principles are enough to explain completely the movement of almost every single object you're ever likely to come across. The first law outlines the concept of inertia. It says that things stay still or move steadily (at the same speed) unless something pushes or pulls them (unless some kind of a force is applied). The second and third laws are of more interest.

The second law explains the connection between force and acceleration: if you push or pull something, it starts moving (if it was still to begin with) or goes faster (if it was moving already); the bigger the force you apply, the more acceleration you get; the longer you apply the force, the bigger the change in momentum you can achieve. Where swimming is concerned, the third law is perhaps the most important. It says that when you apply a force to an object, the object returns the favor and applies an equal force to you—in the opposite direction. This law is often called action and reaction and it's the simplest way for a scientific non-swimmer to make sense of the water. You probably know already that if you kick backward against the wall of a swimming pool, you shoot forward through the water. The same applies to actual swimming strokes. Simply speaking, if you want to swim forward through water, you have to pull water backward with your hands. If you want your body to stay up, floating on the surface, you need to kick down with your legs.

If you're swimming along and you want to stop suddenly and stand up, you can pull your hands down in front of you (in a kind of circular motion—a bit like bowing down) and your legs will swing down behind you, so you land in an upright position on your feet. Master these basic moves—simple applications of Newton's third law—and you'll find you'll be able to swim easily and stop confidently whenever you need to. Photo: Isaac Newton tells us we have to pull water backward to go forward, as this swimmer is doing by using his outstretched hand and forearm as a paddle. Photo by Alan D. Monyelle courtesy of US Navy . Minimizing your drag Lots of other scientific factors make a big difference to how well you can move through the water. Once you've mastered the basic science of swimming, minimizing your drag in the water is the next step: that will help you swim faster and for longer.

Many beginners don't really understand this, but it's exactly the same as cycling: in the same way that cyclists have to minimize the surface area they present to the wind (crouch forward, put their arms in, and generally streamline themselves), so swimmers have to create as little resistance to the water as they possibly can. In practice, this means making your body completely horizontal, so (in the case of front crawl) your head is well down in the water rather than poking up with your body sloping down behind it. (That's why you have to learn how to breathe in at the side and breathe out underwater.) You can also minimize drag by slicing your hand in and out of the water to make your strokes and, in front crawl, you can learn to swivel your body as you swim from side to side. Photo: Speed cyclists realize they have to minimize drag because they can feel the air pushing hard against them. Even though water is "thicker" and swimmers feel the drag of the water much more, they don't always realize the importance of minimizing drag. It's worth noting that sea-water is harder to swim in than pool water, for several reasons.

First, except on beautifully calm summer days, the ocean is almost always more turbulent, so your body doesn't slice through the water like a dolphin. Sea-water is also more dense than freshwater because of the salt it contains, and that makes it slightly more viscous too. And cold water (in the ocean) is more viscous than hot water (in a heated pool); the viscosity of water at 10°C (50°F) is twice that of water at 40°C (~100°F). All these things make a cold ocean swim a tougher proposition than a swim in the heated pool, but the upshot is that your body is working harder and getting more exercise. Unlike with cycling or sprinting through air, it's hard to built up any momentum when you're swimming: the water resistance is constantly trying to bring you to a halt. What we have here is the first law of motion in action. If water were as light as air but you could still float and swim through it, you could stroke for a while and then rest, allowing your momentum to keep you moving forward (much as you can stop pedaling on a bicycle every so often). But the force of the water pushing against you brings you rapidly to a rest.

You'll also experience inertia when you try to change direction: since velocity is speed in a particular direction, changing direction means changing velocity—and it requires you to use a force, even if you swim at constant speed. If you're doing front crawl and you decide you want to turn around in a semi-circle and go back the way you came, it's actually quite hard to change the direction of your motion without stopping and reversing or doing a somersault. Swimming efficiently Swimming is superb aerobic exercise (vigorous exercise that really pumps your heart and lungs) and very tiring; the two things are, of course, connected. You can swim further for longer by swimming more efficiently, which means using as little energy as possible for each stroke by getting as much forward propulsion as possible. With front crawl, the object is to extend your hand as much as you can and bring it back as far as possible, dragging as much water back (with a cupped hand and a bent forearm) as you possibly can. If you make a long, complete stroke with a proper follow-through, you're applying your pulling force for longer and each stroke will count for more.

You can see this from Newton's second law of motion, which is often written: force = mass × acceleration F = m a Since acceleration is velocity divided by time, it's also true that force is equal to the rate of change of momentum: F = mv / t force × time = mass × velocity F t = m v To put it another way, if you want to produce the biggest possible change of momentum, you need to apply your force (pulling back on the water) for as long as possible—with as long a stroke as possible and a good, complete follow-through. It's also worth remembering that the human body is a machine (in the strict scientific sense of that word): our limbs work like levers, pivoted at our joints (which are effectively fulcrums), multiplying force or speed. When you're doing front crawl, it's important to reach forward and pull your arm backward as much as you possibly can. You get more leverage on the water that way and the force you create pulling backward will give you more force to go forward. A good follow-through also decelerates your limbs more slowly, and reducing the acceleration reduces the force they feel, reducing the likelihood of pulled muscles and other injuries.

Photo: It's important to reach forward and extend your arm as much as possible. Photo by Joseph M. Clark courtesy of US Navy . The conservation of momentum tells us that the momentum you give your body, going forward, is the same as the momentum you give the water, pulling backward. That implies that you need to pull as much water backward as you possibly can with each stroke. Cupping your hand helps; so does using your forearm as a kind of paddle, so you pull back an entire arm's worth of water rather than a mere handful. You'll find this is much harder and more tiring to begin with, which is a good sign: it shows you're creating much more force. Energy and power It takes energy to push your body through the water—and your body loses the same amount of energy in the process. The rate at which something uses energy is called power. According to an interesting blog post in Wired by physicist Rhett Allain, champion swimmers can briefly achieve a power of 1200 watts (the maximum power of a clothes washing machine or a very powerful vacuum cleaner), which is about three times what a champion cyclist achieves.

It's not surprising that a swimmer uses more energy than a cyclist given that all four of a swimmer's limbs are moving all the time (compared with just a cyclist's legs), and in a more dense fluid. But it is surprising to me that the difference is so great: there's no obvious reason why a human body working at full tilt should be able to produce so much more power in one fluid (water) than another (air). In his book The Human Machine, British zoology professor R. McNeill Alexander quotes power figures for swimming that are in the hundreds of watts. Consistent with what cyclists produce, that seems more likely to me. (It's also similar to the figure in the calculation in the box at the end of this article.) Chart: Top swimmers (and cyclists) can produce several hundred watts of power, which is about twice as much as the rest of us. This chart shows how that amount of power compares to some familiar everyday appliances. In theory, a swimmer could power three LCD TVs, four old-fashioned lamps, or 20 energy-efficient lamps.

Floating and buoyancy Things float because when we place them in water, the pressure of the water underneath them pushes up and supports them; in other words, water pressure pushing upward balances weight (the force of gravity) pulling downward. That's one of the reasons why we swim in a horizontal position: spreading the body flat makes it work more like a raft, so there's more upthrust from the water below. You probably know that it's much easier to float on your back than standing straight upward, when you need to "tread water" (kick and push your arms downward to create an upward force that stops you sinking). A non-swimmer's biggest fear is sinking under the water and drowning, but it's much harder to sink when you're swimming than you might suppose. (Unlike when you accidentally fall into a river, where you're more likely to sink and drown because your clothes get wet and stop you swimming properly; factors such as the coldness of the water also play a part.) Depending on your body type (how big you are, how much you weigh, how big your lungs are, how fat you are, and so on), you may be surprisingly buoyant: you might find it quite hard to sink even if you want to.

It's fairly well known that fatter people are more buoyant than skinnier ones, and that's because fat is less dense (more buoyant) than muscle. Wearing a wetsuit (made from a synthetic rubber called neoprene, which traps air bubbles inside it) makes you even more buoyant, which is why scuba divers typically have to wear weights to make them sink. Photo: Our bodies are surprisingly buoyant, but we float better in some positions than others. This swimmer is practicing a survival technique called the prone or "dead man's float," which helps you float in water for longer and conserve energy. Photo by William R. Goodwin courtesy of US Navy . Is it better to float or to sink? If you're a boat, it's certainly better to do one or the other! Unfortunately, most boats do a bit of both: they crash and drag straight through the waves—in the very turbulent interface between the air and the water. The fastest boats are hydrofoils and hovercraft (which aim to lift themselves clear of the waves) and submarines (which sink beneath them). If you're a swimmer, neither of these is really an option.

We can't choose whether to sink or float: we have to drag through the water. Even so, understanding the science of swimming and mastering how we apply it can help us poor land creatures to move as efficiently through the water as possible! Question: Does swimming warm the pool? If you're a fan of science, you probably know about one of the greatest physics experiments of all time, which was carried out by James Prescott Joule in 1840. He was proving what's now called the law of conservation of energy —the basic idea that we can't create or destroy energy, but only convert it from one form to another. He did it using this apparatus, in which a weight (1) running over a pulley (2) pulls on a rope (3) and spins a paddle wheel so it agitates water inside a closed container (4), heating it up by a small amount. After repeating the experiment quite a few times to get a measurable temperature rise, Joule calculated that the potential energy lost by the falling weight was exactly the same as the heat energy gained by the water in the container.

Recently someone emailed me asking whether swimmers warm the water they're standing or moving in, which is a more complex question than you might think for all sorts of reasons. But it set me thinking about Joule's experiment and whether a pool full of people, swimming furiously, would warm the water by a noticeable amount. This is the kind of "back-of-envelope" calculation that all physicists love! Let's assume some things to make life easy: As people swim, all the energy their bodies produce in the process ends up as heat in the water. The only thing warming the water is the physical agitation caused by the swimming. I'm going to completely ignore the ordinary heat lost from the swimmer's bodies to the cooler water (mostly by conduction). All the energy that goes into the water stays there. None goes into the air above or the material surrounding the pool. The swimmers can happily keep swimming away at a constant rate indefinitely, no matter how hot the water gets (if indeed it does). There's no reason why the swimmers couldn't be replaced by other people and, if necessary, we could put them in heatproof suits!

The specific heat capacity of water (the amount of energy it takes to raise 1g of water by 1°C) stays the same as the temperature changes. And so on. I'm just interested in a "guesstimate," not an exact calculation. Let's assume that there are 50 swimmers in an Olympic size pool and they swim so fast that each of them consumes 1500 kJ of energy per hour and puts that much heat into the water. (That's a figure I've pulled from p26 of Richard Muller's excellent book Physics for Future Presidents , but it seems to be confirmed elsewhere. It's the equivalent of a few hundred calories.) If 50 swimmers swim for one hour, we get a total heat energy input of 7.5 x 107J. How much does that warm the water? An Olympic-sized pool has a (fairly gigantic) volume of 2.5 million liters, and a liter weighs about a kilogram, so we have a mass of water of 2.5 x 106kg.

The specific heat capacity of water is about 4.2 joules per gram per °C (in other words, it takes 4.2 joules to raise the temperature of 1 gram of water by 1°C). So an energy input of 7.5 x 107J raises the pool temperature by 7.5 x 107 / (4.2 × 1000 to convert kilograms to grams × 2.5 x 106) = 0.007°C! If the water is 20°C to start with, how long would it take the swimmers to make it boil? We'd need a temperature rise of 80°C and we know it takes an hour to give us 0.007°C, so we'd need our swimmers to keep going for 11,200 hours = 466 days = about a year and three months. Answer Does swimming warm the pool? 1. Yes—the energy swimmers generate has to go somewhere. 2. No—the heat they produce by swimming is so minute that it makes no difference whatsoever, and the heat lost from the water would more than make up for it.

Forces that Influence Driver Control Forces that Influence Driver Control MOTION CONTROL Earlier in this program, we discussed man-made laws and how they impact our driving effectiveness. In this segment, we're going to talk about several other laws that come into play when we're driving a vehicle. They're called natural laws. GRAVITY Take gravity, for instance. Gravity is what keeps us from flying off into space--at least that's what it's supposed to do. Just when we get a little cocky, though, gravity has a way of bringing us back down to earth, unless, of course, you're an astronaut. However, it's not a good idea to defy the laws of gravity when you're driving a car. CENTRIFUGAL FORCE Centrifugal force is another natural law we have to contend with when driving. It's the force that pulls an object out from the center. It, too, is a force to be reckoned with when you're driving. INERTIA Then, there's a thing called the law of inertia.

It says that a body in motion tends to stay in motion; that is, if you're in a moving vehicle that stops suddenly, you'll keep right on going at the same rate of speed you were going before the vehicle stopped, even if you don't want to. Look, the dashboard!! � Braking � Steering There are three basic ways that we control the motion of our cars. They are (1) speed, (2) braking, and (3) steering. Each of these functions is affected by the laws of gravity, centrifugal force, inertia, kinetic energy, and friction. ENERGY=SPEED When you accelerate your car, you are increasing the car's energy. Keep in mind, the faster the car is moving, the longer it will take for the vehicle to stop. Your car will respond differently at faster speeds, so you must adjust the way you accelerate, decelerate and steer. A vehicle's energy of motion doubles when its weight doubles. When a vehicle's weight doubles, it needs about twice the distance to stop. A vehicle's energy of motion is proportional to the square of its increased speed. When the vehicle's speed doubles, it needs about four times the distance to stop.

When the vehicle's speed triples, it needs about nine times the distance to stop. All of which means, the faster you're driving, the longer it takes you to stop, and the less time you have to react to a situation. You also have increased difficulty steering in the intended path of travel and an overall decrease in car control. Along with those safety considerations, speeding also causes decreased fuel efficiency, increased engine wear, and increased tire wear. By reducing speeds, or decelerating, you will have more time to react, shorten your braking distance, have less difficulty steering in your intended path of travel, and have an overall increase in car control. You'll enjoy better fuel efficiency, less engine wear, and less wear and tear on your tires. FRICTION If it weren't for friction, you couldn't stop your car in an emergency. You'd have to wait for it to stop itself. Friction is critically affected by your brakes, tires, the road surface, and speed. The brakes on your car have to be strong enough to lock the wheels into a skid. Your tires must have enough tread to create good traction. Traction is the friction of gripping power of the tire as it moves on the roadway.

FACTORS AFFECTING TRACTION Centrifugal force Hydroplaning Traction is affected in one way or another by friction. For instance, consider the effect of these factors: stopping distance, centrifugal force, and/or hydroplaning. An increase in any or all of these factors will decrease the amount of traction that your car will have because there is a corresponding decrease in the opportunity for friction. By decreasing a car's traction, you are thereby decreasing the amount of control you have over the vehicle. When your car goes into a full skid, you don't get maximum braking. Sometimes a driver will slam on his brakes and slide right into the object he was trying to avoid. When you're in a full skid, your front wheels are sliding and you lose control over the steering. The direction of your car won't change, even if you turn the steering wheel. Sometimes you can regain steering control by releasing the brake enough so the wheels are not skidding, then steering around the object you're trying to avoid hitting. Tires with good tread are necessary for good traction on our roadways. The purpose of tread is to increase frictional contact with the road to give us greater stopping power.

Good friction is required for proper traction on the roadway--so is a good road surface. The type of road surface greatly influences the amount of friction our tires create with the roadway. A dry, concrete road provides the best friction, while dirt roads are one of the worst. On dirt roads, our tires are literally rolling across small stones that have a ball bearing effect. Needless to say, you need a lot more room to stop on a dirt road than you do on dry pavement. On wet pavement, most of the friction is lost due to hydroplaning. The mixture of oil and water on a concrete or asphalt road causes the tires to "float" on an emulsion. The tires, in effect, lose their contact with the pavement, and thus, lose the benefit of friction. However, the most difficult surface for maintaining any type of friction is on an icy road. It will take you three times farther to stop than under ordinary conditions. SPEED AND KINETIC ENERGY Keep in mind also that every time you stop your car, you have to overcome a force that is known as kinetic energy. This force is the energy developed by an object while it's in motion.

In very simple terms, the law of kinetic energy means that when you double your speed, the kinetic energy of your car is four times as great! And that means the braking distance is four times as great. When you triple or quadruple your speed, the stopping distance is even greater. Let's say you're driving at 20 mph on dry concrete pavement. At that rate of speed, you'll need 20 feet to stop your car, or one foot for each mile per hour. Now you've doubled your speed to 40 mph. You'll need four times the stopping distance, or 80 feet, to bring your car to a safe stop. So you can see, kinetic energy has a direct bearing on speed versus stopping distance. CENTRIFUGAL FORCE Another force you need to understand is centrifugal force. A simple definition is the force that pulls an object out from the center of a curve. Centrifugal force comes into play when you're rounding a curve. It tends to pull your vehicle out away from the inside edge, regardless of which direction you're traveling.

If a car on the "inside" lane of a curve tries to navigate with too much speed, centrifugal force will pull it away from the center and, potentially, into the path of oncoming traffic. Or, if a car is traveling with too much speed from the other direction--in the "outside" lane of a curve--centrifugal force will pull it out to the right and off the roadway. If you would like to see the animation again, right click on the animation and choose "Rewind" from the menu list. INERTIA Anybody who has ever survived a serious car crash can tell you about this force. If a vehicle is traveling at 40 mph and hits an object that stops its motion, everybody and everything in that vehicle keeps traveling at 40 mph until they hit something that stops them. This is the force called inertia. Inertia is the force that causes the second and third collisions in an automobile crash. Bodies hitting bodies, and objects in the car hitting bodies. In this test, a driver is going 40 mph without his safety belt on. His vehicle hits a stationary object and stops. But he keeps moving at 40 mph--until he hits the windshield, that is.

GRAVITY Gravity is another force that affects car handling. It is a force that pulls objects downward. When a vehicle is going downhill, gravity is one of the forces pulling it down. Conversely, when your vehicle is going uphill, gravity helps slow it down. FORCES AFFECTING CONTROL AND VEHICLE PERFORMANCE INERTIA CENTRIFUGAL FORCE GRAVITY Inertia, kinetic energy, centrifugal force, and gravity all affect driver control and vehicle performance. They affect stopping distance, car control, and the force of an impact. � Stopping Distance FACTORS AFFECTING IMPACT SPEED Speed is one of the factors affecting impact. When you increase the speed at which you hit an object, you directly increase the force of the impact. When you increase the force of impact, you increase the risk of injury or death in a collision. WEIGHT When you increase the weight of an object, whether it is the vehicle you are driving or the object you are about to hit, you increase the force of the impact, and conversely, the risk of injury or death.

DISTANCE When you increase the distance your car has to travel between the time you put on the brakes and the time you reach impact, you increase the ability of your car to slow down before reaching the point of impact. You are, therefore, decreasing the force of impact, which decreases the risk of injury or death in a collision. GREATER SPEED AND WEIGHT: Inertia and Mass Newton's Laws - Lesson 1 - Newton's First Law of Motion Inertia and Mass Balanced and Unbalanced Forces Newton's first law of motion states that "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Objects tend to "keep on doing what they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia. Inertia: the resistance an object has to a change in its state of motion . Newton's conception of inertia stood in direct opposition to more popular conceptions about motion. The dominant thought prior to Newton's day was that it was the natural tendency of objects to come to a rest position.

Moving objects, so it was believed, would eventually stop moving; a force was necessary to keep an object moving. But if left to itself, a moving object would eventually come to rest and an object at rest would stay at rest; thus, the idea that dominated people's thinking for nearly 2000 years prior to Newton was that it was the natural tendency of all objects to assume a rest position. Galileo and the Concept of Inertia Galileo, a premier scientist in the seventeenth century, developed the concept of inertia. Galileo reasoned that moving objects eventually stop because of a force called friction. In experiments using a pair of inclined planes facing each other, Galileo observed that a ball would roll down one plane and up the opposite plane to approximately the same height. If smoother planes were used, the ball would roll up the opposite plane even closer to the original height. Galileo reasoned that any difference between initial and final heights was due to the presence of friction. Galileo postulated that if friction could be entirely eliminated, then the ball would reach exactly the same height. Galileo further observed that regardless of the angle at which the planes were oriented, the final height was almost always equal to the initial height.

If the slope of the opposite incline were reduced, then the ball would roll a further distance in order to reach that original height. Galileo's reasoning continued - if the opposite incline were elevated at nearly a 0-degree angle, then the ball would roll almost forever in an effort to reach the original height. And if the opposing incline was not even inclined at all (that is, if it were oriented along the horizontal), then ... an object in motion would continue in motion... . Forces Don't Keep Objects Moving Isaac Newton built on Galileo's thoughts about motion. Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a rest position. The book in motion on the table top does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - that brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.) A force is not required to keep a moving book in motion.

In actuality, it is a force that brings the book to rest . Mass as a Measure of the Amount of Inertia All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion. Suppose that there are two seemingly identical bricks at rest on the physics lecture table. Yet one brick consists of mortar and the other brick consists of Styrofoam. Without lifting the bricks, how could you tell which brick was the Styrofoam brick? You could give the bricks an identical push in an effort to change their state of motion. The brick that offers the least resistance is the brick with the least inertia - and therefore the brick with the least mass (i.e., the Styrofoam brick).

A common physics demonstration relies on this principle that the more massive the object, the more that object resist changes in its state of motion. The demonstration goes as follows: several massive books are placed upon a teacher's head. A wooden board is placed on top of the books and a hammer is used to drive a nail into the board. Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia). This is demonstrated by the fact that the teacher does not feel the hammer blow. (Of course, this story may explain many of the observations that you previously have made concerning your "weird physics teacher.") A common variation of this demonstration involves breaking a brick over the teacher's hand using the swift blow of a hammer. The massive bricks resist the force and the hand is not hurt. (CAUTION: do not try these demonstrations at home.) A physics instructor explains the property of inertia using a phun physics demonstration. Check Your Understanding 1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will a. gradually stop.

See Answer According to Newton's first law, the rock will continue in motion in the same direction at constant speed. 2. A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction? Answer: 0 N An object in motion will maintain its state of motion. The presence of an unbalanced force changes the velocity of the object. 3. Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why. See Answer Tosh is correct. Inertia is that quantity which depends solely upon mass. The more mass, the more inertia. Momentum is another quantity in Physics which depends on both mass and speed. Momentum will be discussed in a later unit. 4. Supposing you were in space in a weightless environment, would it require a force to set an object in motion? Absolutely yes! Even in space objects have mass. And if they have mass, they have inertia.

That is, an object in space resists changes in its state of motion. A force must be applied to set a stationary object in motion. Newton's laws rule - everywhere! 5. Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What affect (if any) does this practice have upon his inertia? Explain. Fred's inertia will increase! Fred will increase his mass if he makes a habit of this. And if his mass increases, then his inertia increases. 6. Ben Tooclose is being chased through the woods by a bull moose that he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion. See Answer The large mass of the bull moose means that the bull moose has a large inertia. Thus, Ben can more easily change his own state of motion (make quick changes in direction) while the moose has extreme difficulty changing its state of motion. Physics for better living! 7.

Two bricks are resting on edge of the lab table. Shirley Sheshort stands on her toes and spots the two bricks. She acquires an intense desire to know which of the two bricks are most massive. Since Shirley is vertically challenged, she is unable to reach high enough and lift the bricks; she can however reach high enough to give the bricks a push. Discuss how the process of pushing the bricks will allow Shirley to determine which of the two bricks is most massive. What difference will Shirley observe and how can this observation lead to the necessary conclusion? See Answer The bricks, like any object, possess inertia. That is, the bricks will resist changes in their state of motion. If Shirley gives them a push, then the bricks will offer resistance to this push. The one with the most mass will be the one with the most inertia. This will be the brick which offers the most resistance. This very method of detecting the mass of an object can be used on Earth as well as in locations where gravitational forces are negligible for bricks.

Question: What is the force that brings moving bodies to a halt called
Answer: Friction